"We will also respond when nations retreat from democracy, as in Thailand. We urge the Royal Thai Armed Forces to release those who have been detained, end restrictions on free expression, and move immediately to restore power to the people of Thailand through free and fair elections.

"Until that happens, as US law requires, the US State of Department is suspending and reconsidering US assistance and engagement with Bangkok", he said.

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Chaturon Chaisang waves from inside a military van after being detained. Credit: Reuters

Soldiers have captured and detained Thailand's former education minister, an open critic of the new military regime in the country.

Chaturon Chaisang, who had been on the run after refusing to turn himself in to the military after being summoned, had just given a talk at the Foreign Correspondents' Club of Thailand.

He had denounced a coup that he said would only exacerbate conflict in the polarised country.

Other former ministers were detained by the military after the coup on May 22 although most, including former premier Yingluck Shinawatra, have now been freed, even if their movements are being monitored.

Thai soldiers take up position to secure the Victory Monument square to bar protesters from staging anti-coup demonstration in Bangkok. Credit: AP Photo/Wason Wanichakorn

Thailand's military ruler has issued a strong warning to anyone opposed to last week's coup, urging dissenters not to cause trouble or else the nation could rever back to the "old days" of turmoil and street violence.

Speaking in his first public appearance since seizing power, General Prayuth Chan-ocha, said:

"I'm not here to argue with anyone. I want to bring everything out in the open and fix it.

Right now there are people coming out to protest. So do you want to go back to the old days? I'm asking the people in the country, if you want it that way, then I will have to enforce the law."

Thai Army chief General Prayuth Chan-ocha looks on during a news conference at Army Headquarters in Bangkok on Monday. Credit: Reuters

Coup leader General Prayuth Chan-ocha has said that Thailand's king had formally endorsed his position as head of the military council that will run the country.

He also said in a statement he read on television that he would have no choice but to use force if political protests flared up again. He said he aimed for an election as soon as possible but gave no timeframe for a vote.

The army imposed martial law on May 20 and then seized power on May 22 after nearly seven months of political turmoil.

The US State Department has cancelled engagements with Thailand amid a military coup, it said in a statement.

People hold signs while kneeling during a protest against military rule at Victory Monument in central Bangkok. Credit: REUTERS/Athit Perawongmetha

"We urge the immediate restoration of civilian rule and release of detained political leaders, a return to democracy through early elections, and respect for human rights and fundamental freedoms", the department said.

The US military has cancelled training exercises with Thailand, as well as a visit to the Asian nation.

Pentagon Press Secretary Real Admiral John Kirby said: "As we have made clear, it is important that the Royal Thai Armed Forces end this coup and restore to the people of Thailand both the principles and the process of democratic rule, including a clear path forward to elections."